The sequence valve and pressure-reducing valve have some similar features. It is instructive to compare their symbols side by side (Fig. 3.20). Both valves are externally drained, meaning that there is a separate line from the valve back to the reservoir. The relief valve and unloading valve are both internally drained. They have a passageway machined into the housing, which allows leakage from the spring cavity to flow to the outlet. Since the outlet is connected to the reservoir, a separate line is not needed. The outlets of the sequence and pressure-reducing valves are not connected to the reservoirs, thus they cannot be internally drained.
The check valve built into both valves is there to provide free flow in the reverse direction. In effect, it takes the valve out of the circuit when flow is reversed.
As shown in Fig. 3.20, the sequence valve and pressure-reducing valve symbols are similar. One (sequence valve) is a normally closed valve with pilot line to sense inlet pressure, and the other (pressure-reducing valve) is a normally open valve with pilot line to sense outlet pressure.